Hydrophilic group-terminated fluorine-containing vinyl ethers can be used, either as such or after protection of the terminal hydrophilic group by fluorination or by esterification, amidation or imidation, for the production of copolymers by polymerization with another fluoroolefin and/or the like.
The copolymers obtained can have salt-forming hydrophilic groups and, therefore, the use thereof as ion exchange membranes in salt electrolysis, chemical sensors, separation membranes, fuel cells and so forth is under investigation. They can also be utilized as superstrong acid catalysts in the form of powders as such, or in lithium cells and so on.
As a method for obtaining water-soluble fluorine-containing vinyl ethers, there is disclosed, in WO 98/43952 pamphlet, a thermal decomposition method which comprises heating, for decarboxylation, a fluorine-containing 2-alkoxypropionic acid derivative with carboxyl group being in the form of a metal salt. This thermal decomposition method known in the art has a problem in that a side reaction occurs, namely oligomers are formed from the fluorine-containing 2-alkoxypropionic acid derivative.
As regards the reaction conditions for this thermal decomposition, WO 01/28989 pamphlet discloses that the reaction should preferably be carried out at 170 to 230° C. using 1 to 5 parts by weight of a catalyst having a coordinating property with a metal ion per 100 parts by weight of the fluorine-containing 2-alkoxypropionic acid derivative.
Under such conditions, however, the byproduct oligomers are formed in large amounts and the yield of the desired product thus decreases. In carrying out the reaction on a large scale using a large-sized reactor, in particular, a long time is required for raising the temperature and for lowering the temperature, whereby the production of oligomers further increases. Thus, there is a problem that such conditions are difficult to apply on a commercial scale.
The water-soluble fluorine-containing vinyl ethers, which are the target products to be obtained by this thermal decomposition reaction, occur as high-boiling salts and, therefore, the distillation method cannot be used as the method of purifying them. A promising method is recrystallization. However, it is not easy to crystallize the water-soluble fluorine-containing vinyl ethers in the presence of a large amount of oligomers, which are uncrystallizable and thus render the purification by recrystallization difficult to achieve.